Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210901
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
401 AM CDT MON APR 21 2014

.DISCUSSION...
305 AM CDT

MAIN FORECAST CONCERN IS WITH RAIN/THUNDER CHANCES AND TRENDS TODAY
WITH COLD FRONT MOVING THROUGH THE FORECAST AREA THIS AFTERNOON AND
EARLY THIS EVENING. PATTERN REMAINS AMPLIFIED AND PROGRESSIVE
THROUGH THE WEEK...WITH ANOTHER ORGANIZED CHANCE OF PRECIP AND A
FRONTAL PASSAGE THURSDAY AND YET ANOTHER LATER NEXT WEEKEND.

EARLY MORNING SURFACE MAP DEPICTS A TROUGH OF LOW PRESSURE EXTENDING
FROM NORTHWEST MINNESOTA SOUTHWEST THROUGH THE SIOUX FALLS AREA AND
INTO THE CENTRAL PLAINS. A COMPLEX UPPER TROUGH WAS OVER CENTRAL
NORTH AMERICA WITH AT LEAST THREE DISTINCT SHORT WAVE DISTURBANCES
FROM NORTH TO SOUTH PER 00Z RAOB AND CURRENT GOES VAPOR IMAGERY. THE
UPPER TROUGH IS PROGGED TO CONTINUE TO PROPAGATE EAST ACROSS THE
AREA TODAY...WITH SURFACE LOW PRESSURE LIFTING NORTH OF THE CWA AND
DEEPENING ACROSS THE NORTHERN GREAT LAKES IN RESPONSE TO THE
STRONGER NORTHERN STREAM SHORT WAVE DIGGING ALONG THE US/CANADIAN
BORDER. LOW LEVEL MOISTURE PLUME FROM TEXAS TO THE UPPER MISSISSIPPI
VALLEY HAS BEEN LIMITED SOMEWHAT BY PIECEMEAL NATURE OF UPPER
FORCING ALONG UPPER TROUGH AND BLOCKING OF CENTRAL/EASTERN GULF BY
EXPANSIVE SURFACE HIGH PRESSURE RIDGE WHICH EXTENDS FROM THE LOWER
MISSISSIPPI VALLEY TO OFF THE NEW ENGLAND COAST. HOWEVER...STARTING
TO SEE DEVELOPMENT OF ISOLATED SHRA/TSRA INTO FAR NORTHERN
IL/SOUTHERN WI EARLY THIS MORNING IN ADVANCE OF APPROACHING SHORT
WAVE AND ABOVE SUBTLE ELEVATED WARM FRONTAL ZONE WHICH STRETCHES
EAST/SOUTHEAST FROM SIOUX FALLS LOW. GUIDANCE RUNS IN GENERAL
AGREEMENT THAT PRECIP CHANCES WILL SLOWLY EXPAND ACROSS THE REGION
BY THIS AFTERNOON AS SURFACE COLD FRONT TRACKS EAST INTO THE AREA AS
SURFACE LOW DEEPENS TO OUR NORTH. COMBINATION OF LARGE SCALE ASCENT
ASSOCIATED WITH MID-LEVEL TROUGH AND LOW LEVEL FRONTAL CONVERGENCE
RESULTS IN STEEPENING LAPSE RATES BY AFTERNOON...WITH FORECAST
SOUNDINGS DEPICTING ABOUT 300-700 J/KG MLCAPE IN PRE-COLD FRONTAL
ENVIRONMENT. THIS IS EXPECTED TO BE SUFFICIENT TO PRODUCE INCREASING
COVERAGE OF SHRA/TSRA AS THE FRONT MOVES EAST ACROSS THE REGION THIS
AFTERNOON AND EARLY THIS EVENING...THOUGH DEEP SHEAR REMAINS
RELATIVELY WEAK WITH MAIN UPPER JET CORE WELL TO OUR NORTH. PRECIP
CHANCES THEN DIMINISH RAPIDLY FROM THE WEST/NORTHWEST THIS EVENING
AS FRONT PULLS AWAY TO THE EAST AND DRIER AIR PUSHES IN FROM THE
NORTHWEST. LOW LEVEL THERMAL RIDGE AHEAD OF COLD FRONT SHOULD EASILY
SUPPORT TEMPS IN THE 70S AGAIN TODAY...WITH SOUTHWEST BOUNDARY LAYER
FLOW 10-15 KT ENOUGH TO PREVENT ANY SIGNIFICANT LAKE BREEZE
DEVELOPMENT.

STRONG SHORT WAVE DIGS SOUTHEAST ACROSS THE GREAT LAKES REGION
TONIGHT...WITH COOL/DRY LOW LEVEL ADVECTION IN THE WAKE OF
ASSOCIATED SURFACE LOW AND COLD FRONT RESULTING IN COOLER TEMPS
TONIGHT AND TUESDAY...WITH NORTHERLY WINDS AHEAD OF SURFACE HIGH
PRESSURE WHICH BUILDS INTO THE AREA TUESDAY NIGHT AND WEDNESDAY.
WHILE IT WILL BE SUNNY...AFTER A COUPLE OF DAYS WELL INTO THE
70S...LOWER 60S APPEARS TO BE ABOUT THE WARMEST ACROSS OUR FAR
WESTERN/SOUTHERN COUNTIES ON TUESDAY AND WEDNESDAY WHILE NORTHEAST
WINDS OFF LAKE MICHIGAN LIMIT CHICAGO METRO AREA TO MID-UPPER
50S...WITH SOME 40S RIGHT ALONG THE LAKE. LOWS MAY DIP INTO THE LOW
30S IN SOME SPOTS TUESDAY NIGHT AS WINDS DIMINISH WITH THE APPROACH
OF THE RIDGE AXIS.

MEDIUM RANGE FORECASTS REMAIN CONSISTENT IN BRINGING ANOTHER RATHER
AMPLIFIED UPPER TROUGH EAST OF THE ROCKIES ON WEDNESDAY...WITH
SURFACE LOW PRESSURE DEVELOPING ACROSS THE PLAINS. MOISTURE RETURN
LOOKS TO BE A LITTLE MORE ROBUST WITH THIS FEATURE...WITH MOISTENING
AND SATURATION IN ISENTROPIC WARM AIR ADVECTION POTENTIALLY YIELDING
SOME SHOWERS ACROSS PARTS OF NORTHWEST IL BY WEDNESDAY AFTERNOON...
AND ACROSS MUCH OF THE FORECAST AREA WEDNESDAY NIGHT. THE GREATEST
CHANCE FOR PRECIP IS THURSDAY...AS OCCLUDING COLD FRONT PLOWS EAST
ACROSS THE CWA. WHILE TEMPS WARM...POSITION OF TRIPLE POINT IN
OCCLUSION REMAINS ACROSS/SOUTH OF OUR AREA AND DOES NOT PORTENT
TEMPS AS WARM AS THIS PAST WEEKEND WITH GENERALLY 60S ANTICIPATED
AND PERHAPS ONLY 50S ALONG NORTH SHORE IN SOUTHEAST FLOW. MODELS
DEPICT SECOND DISTURBANCE ROTATING AROUND UPPER LOW TO OUR NORTH
THURSDAY NIGHT INTO FRIDAY...WRAPPING A SECONDARY COLD FRONT
SOUTHEAST ACROSS THE AREA AND MAINTAINING SOME POTENTIAL FOR SHOWERS
ACROSS ESPECIALLY NORTHERN/EASTERN PARTS OF THE FORECAST AREA. SOME
DIFFERENCES ARISE WITH ORIENTATION OF EASTERN NORTH AMERICAN TROUGH
WHICH RESULTS BY NEXT WEEKEND... THOUGH LOOKS TO BE ANOTHER COOLER
THAN NORMAL PERIOD WITH NORTH/NORTHEAST FLOW FOR US TO THE WEST OF
THE UPPER TROUGH.
WHILE RUN TO RUN CONSISTENCY EXISTS IN GFS AND ECMWF LONG RANGE
SOLUTIONS...ECMWF REMAINS FASTER WITH DEPARTING EAST COAST LONG WAVE
TROUGH AND BRINGS ANOTHER LOW/WARM FRONT INTO THE REGION (BUT TO OUR
SOUTH) LATE SATURDAY AND SUNDAY. HAVE GENERALLY FOLLOWED THE ECMWF
TREND WHICH BRINGS PRECIP CHANCE BACK IN NORTH OF THE WARM FRONT
SATURDAY NIGHT/SUNDAY.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* SCATTERED SHOWERS/THUNDERSTORMS DEVELOPING THIS AFTERNOON.

RODRIGUEZ

//DISCUSSION...UPDATED 06Z...

QUIET CONDITIONS TONIGHT/EARLY MONDAY MORNING...WITH MID/HIGH
LEVEL CLOUD COVER CONTINUING TO STREAM OVERHEAD AND SOUTH TO
SOUTHWEST WINDS REMAINING IN PLACE. EXPECT WINDS TO BECOME MORE
SOUTHWEST BY MID MORNING MONDAY WHILE SPEEDS INCREASE THROUGH MID
DAY AND THE AFTERNOON. EXPECT CLOUD COVER TO INCREASE WITH VFR
CEILINGS LIKELY BY MID DAY...AS SCATTERED SHOWERS BEGIN MOVING
ACROSS THE TERMINALS. THIS SCATTERED DEVELOPMENT WILL LIKELY
PERSIST IN THE AFTERNOON UNTIL BETTER DEVELOPMENT ARRIVES OUT
AHEAD OF APPROACHING FRONT LATE IN THE AFTERNOON AND EARLY
EVENING. ITS DURING THIS TIME THAT BEST THUNDER POTENTIAL WILL BE
BEFORE PRECIP QUICKLY EXITS TO THE EAST WITH FROPA...AND WITH
SOUTHWEST TO WEST WINDS TURNING TO THE NORTHWEST.

RODRIGUEZ

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* MEDIUM HIGH CONFIDENCE WITH WIND SPEED/DIRECTION.

* MEDIUM HIGH CONFIDENCE WITH SHOWER/THUNDERSTORM
  POTENTIAL...MEDIUM CONFIDENCE OF TIMING/DURATION.

RODRIGUEZ

//OUTLOOK FOR ORD/MDW FOR 12Z TUESDAY-12Z SUNDAY...UPDATED 00Z...

TUESDAY AND WEDNESDAY...VFR.

THURSDAY...VFR EARLY...THEN A PERIOD OF SHRA/TSRA LIKELY LATE.

FRIDAY AND SATURDAY...MVFR CIGS POSSIBLE.

IZZI

&&

.MARINE...
125 PM CDT

LOW PRESSURE TRACKING ACROSS NORTHERN ONTARIO...WILL PUSH A
FRONTAL BOUNDARY ACROSS THE NORTHERN PORTION OF THE LAKE TONIGHT
INTO MONDAY. THIS WILL SET UP A PERIOD OF LIGHT AND VARIABLE
WINDS THERE THROUGH MONDAY. A SECOND AREA OF LOW PRESSURE WILL
MOVE ALONG THIS FRONT LATE MONDAY...AND ONCE THIS LOW MOVES EAST
OF THE LAKE...A PERIOD OF STRONGER NORTHERLY WINDS WILL SET UP
ACROSS THE LAKE MONDAY NIGHT INTO TUESDAY...BEFORE HIGH PRESSURE
BUILDS OVER THE LAKE WEDNESDAY NIGHT. THESE STRONGER NORTHERLY
WINDS COULD RESULT IN SMALL CRAFT ADVISORY CONDITIONS WITH HIGH
WAVES INTO THE NEAR SHORE WATERS FOR TUESDAY.

THE LATER PORTION OF THE WEEK APPEARS TO REMAIN ACTIVE
ACROSS THE GREAT LAKES REGION. IT APPEARS THAT ANOTHER SIGNIFICANT
AREA OF LOW PRESSURE WILL IMPACT THE UPPER MIDWEST AND THE GREAT
LAKES REGION LATE IN THE WEEK AND INTO EARLY NEXT WEEKEND. THIS
SYSTEM COULD PRODUCE PERIODS OF THUNDERSTORMS ACROSS THE LAKE AND
ALSO ADDITIONAL PERIODS OF SIGNIFICANT WINDS.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...10 PM MONDAY TO 10 AM TUESDAY.

&&

$$

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